1. Field of the Invention
The present invention relates to a compound objective lens composed of an objective lens and a hologram lens which has two focal points, an imaging optical system for converging light on two converging spots placed at different depths of an information medium with the compound objective lens, an optical head apparatus for recording, reproducing or erasing information on or from an information medium such as an optical medium or a magneto-optical medium like an optical disk or an optical card with the imaging optical system, an optical disk in which a series of high density recording pits and a series of comparatively low density recording pits are provided, an optical disk apparatus for recording or reproducing information on or from the optical disk with the compound objective lens, a binary focus microscope having two focal points in which two types of images drawn at different depths are simultaneously observed, and an alignment apparatus for aligning two types of images drawn at different depths with the binary focus microscope.
2. Description of the Related Art
An optical memory technique has been put to practical use to manufacture an optical disk in which a pit pattern formed of a series of pits is drawn to record information. The optical disk is utilized as a high density and large capacity of information medium. For example, the optical disk is utilized for a digital audio disk, a video disk, a document file disk, and a data file disk. To record information on the optical disk and to reproduce the information from the optical disk, a light beam radiated from a light source is minutely converged in an imaging optical system, and the light beam minutely converged is radiated to the optical disk through the imaging optical system. Therefore, the light beam is required to be reliably controlled in the imaging optical system with high accuracy.
The imaging optical system is utilized for an optical head apparatus in which a detecting system is additionally provided to detect the intensity of the light beam reflected from the optical disk. Fundamental functions of the optical head apparatus are classified into a converging performance for minutely converging a light beam to form a diffraction-limited micro-spot of the light beam radiated on the optical disk, a focus control in a focus servo system, a tracking control in a tracking serve system, and the detection of pit signals (or information signals) obtained by radiating the light beam on a pit pattern of the optical disk. The fundamental function of the optical head apparatus is determined by the combination of optical sub-systems and a photoelectric transfer detecting process according to a purpose and a use. Specifically, an optical head apparatus in which a holographic optical element (or hologram) is utilized to minimize and thin the optical head apparatus has been recently proposed.
2.1. Previously Proposed Art
FIG. 1 is a constitutional view of a conventional optical head apparatus proposed in Japanese Patent Application No. 46630 of 1991 which is applied by inventors of the present invention.
As shown in FIG. 1, a conventional optical head apparatus 11 for recording or reproducing information on or from an information medium 12 such as an optical disk is provided with a light beam source 13 such as a semiconductor laser, a transmission type of blazed hologram 14 for transmitting a light beam L1 radiated from the light beam source 12 without any diffraction in an outgoing optical path and diffracting a light beam L2 reflected on the information medium 12 in a returning optical path, an objective lens 15 for converging the light beam L1 transmitting through the hologram 13 on the information medium 14 to read the information, an actuator 16 for integrally moving the objective lens 15 with the blazed hologram 13 to focus the light beam L1 on the information medium 12 with the objective lens 15, and a photo detector 17 for detecting the intensity of the light beam L2 reflected on the information medium 12 to reproduce the information.
As shown in FIG. 2A, a relative position between the blazed hologram 14 and the objective lens 15 is fixed by a fixing means 18. Or, as shown in FIG. 2B, a blazed pattern is formed on a side of the objective lens 15 to integrally form the blazed hologram 14 with the objective lens 15.
In the above configuration, a light beam L1 (or a laser beam) radiated from the light beam source 13 is radiated to the blazed hologram 14, and the light beam L1 mainly transmits through the blazed hologram 14 without any diffraction in an outgoing optical path. The light beam L1 transmitting through the blazed hologram 14 is called zero-order diffracted light. Thereafter, the zero-order diffracted light L1 is converged on the information medium 12 by the objective lens 15. In the information medium 12, information indicated by a series of patterned pits is recorded and read by the zero-order diffracted light L1. Thereafter, a beam light L2 having the information is reflected toward the objective lens 15 in a returning optical path and is incident to the blazed hologram 14. In the blazed hologram 14, the light L2 is mainly diffracted. The light L2 diffracted is called first-order diffracted light. Thereafter, the first-order diffracted light L2 is received in the photo detector 17.
In the photo detector 17, the intensity distribution of the first-order diffracted light L2 is detected. Therefore, a servo signal for adjusting the position of the objective lens 15 by the action of the actuator 16 is obtained. Also, the intensity of the first-order diffracted light L2 is detected in the photo detector 17. Because the information medium 12 is rotated at high speed, the patterned pits radiated by the light 17 are changed so that the intensity of the first-order diffracted light L2 detected is changed. Therefore, an information signal indicating the information recorded in the information medium 12 is obtained by detecting the change in intensity of the first-order diffracted light L2.
In the above operation, a part of the light beam L1 is necessarily diffracted in the blazed hologram 14 when the light beam L1 is radiated to the blazed hologram 14 in the outgoing optical path. Therefore, unnecessary diffracted light such as first-order diffracted light and minus first-order diffracted light necessarily occurs. In cases where the hologram 14 is not blazed, the unnecessary diffracted light in the outgoing optical path also reads the information recorded in the information medium 12, and the unnecessary light is undesirably received in the photo detector 17. To prevent the unnecessary light from transmitting to the information medium 12, the blazed hologram 14 is manufactured to form a blazed hologram pattern on the surface thereof, so that the intensity of the unnecessary light received in the photo detector 17 is decreased.
Also, because an objective lens of a conventional microscope has only a focal point, images placed within a focal depth of the objective lens can be only observed with the conventional microscope.
Also, a minute circuit is formed on a semiconductor such as a group III-V compound semiconductor to form a microwave circuit, an opto-electronic detector or a solid state laser. In this case, a photo-sensitive material is coated on a sample made of the semiconductor. Thereafter, a relative position between the sample and a photo mask covering the sample is adjusted by utilizing an alignment apparatus, and the photo-sensitive material is exposed by a beam of exposure light through the photo mask to transfer a circuit pattern drawn on the photo mask to the photo-sensitive material in an exposure process by utilizing an exposure apparatus. For example, an alignment pattern is drawn on a reverse side of the sample, and a relative position between the sample and the photo mask is adjusted with high accuracy while simultaneously observing the alignment pattern of the sample and the circuit pattern of the photo mask with the conventional microscope. Thereafter, the circuit pattern of the photo mask is transferred to a front side of the sample.
In this case, because images placed within a focal depth of an objective lens utilized in the conventional microscope can be only observed with the conventional microscope, it is required to utilize the conventional microscope having a deep focal depth in the alignment apparatus in cases where the alignment pattern and the circuit pattern are simultaneously observed with the conventional microscope. Therefore, the magnification of the conventional microscope having a deep focal depth is lowered.
2.2. Problems to be Solved by the Invention
An optical disk having a high density memory capacity has been recently developed because of the improvement in a design technique of an optical system and the shortening of the wavelength of light radiated from a semiconductor laser. For example, a numerical aperture at an optical disk side of an imaging optical system in which a light beam converged on an optical disk is minutely narrowed in diameter is enlarged to obtain the optical disk having a high density memory capacity. In this case, the degree of aberration occurring in the imaging optical system is increased because an optical axis of the system tilts from a normal line of the optical disk. As the numerical aperture is increased, the degree of the aberration is enlarged. To prevent the increase of the numerical aperture, it is effective to thin the thickness of the optical disk. The thickness of the optical disk denotes a distance from a surface of the optical disk (or an information medium) radiated by a light beam to an information recording plane on which a series of patterned pits are formed.
FIG. 3 shows a relationship between the thickness of the optical disk and the numerical aperture on condition that the tilt of the optical axis is constant.
As shown in FIG. 3, because the numerical aperture is 0.5 when the thickness of the optical disk is 1.2 mm, it is effective to thin the optical disk to 0.6 mm in thickness when the numerical aperture is increased to 0.6. In this case, even though the numerical aperture is increased on condition that the tilt of the optical axis is not changed, the degree of the aberration is not increased. Therefore, it is preferred that the thickness of the optical disk be thinned to obtain the optical disk having a high density memory capacity.
Accordingly, it is expected that the thickness of a prospective optical disk having a high density memory capacity becomes thinner than that of a present optical disk such as a compact disk appearing on the market now. For example, the thickness of the compact disk is about 1.2 mm, and the thickness of the prospective optical disk is expected to range from 0.4 mm to 0.8 mm. In this case, it is required to record or reproduce information on or from an optical disk with an optical head system regardless of whether the optical disk is the present optical disk or the prospective optical disk having a high density memory capacity. That is, an optical head apparatus having an imaging optical system in which a light beam is converged on an optical disk within the diffraction limit regardless of whether the optical disk is thick or thin is required.
However, in a conventional optical head apparatus, a piece of information is only recorded or reproduced on or from an optical disk having a fixed thickness. For example, in cases where the thickness of the information medium 12 is off a regular range by about ±0.1 mm or more, an aberration such as a spherical aberration occurs when the optical head apparatus 11 is operated. Therefore, the recording or the reproduction of the information is impossible. Accordingly, there is a drawback that an optical head apparatus in which a piece of information is recorded or reproduced on or from an optical disk regardless of whether the optical disk is the present optical disk or the prospective optical disk having a high density memory capacity cannot be manufactured in a conventional technique.
Also, there is a problem in the conventional microscope. That is, because an objective lens of the conventional microscope has only a focal point and images placed within a focal depth of the objective lens can be only observed with the conventional microscope, the magnification of the conventional microscope and an observed range in an optical axis direction are in a trade-off relationship. Therefore, there is a drawback that it is impossible to observe the images over a wide observed range in the optical axis direction at high magnification
Also, there is a problem in the alignment apparatus. That is, when a circuit pattern drawn on the photo mask is transferred to the front side of the sample after an alignment pattern is drawn on the reverse side of the sample, the alignment of the photomask and the sample is performed by simultaneously observing the circuit pattern of the photo mask and the alignment pattern of the sample with the conventional microscope having a deep focal depth and a low magnification. Therefore, because the conventional microscope has a low magnification, there is a drawback that it is impossible to align the photo mask with the sample at a high accuracy ranging within 5 μm.